Dielectric constant of MgO tunnel barrier with epitaxial strain

Abstract MgO tunnel barrier is a key material in spintronics. MgO-based magnetic tunnel junctions (MTJs) are widely used in magnetic sensors and magnetoresistive random access memory (MRAM) applications. The MgO tunnel barrier also plays a central role in research on the voltage control of magnetic anisotropy (VCMA) effect, which enables an emerging magnetization control technique with ultralow power consumption. However, the value of the dielectric constant ( ε r ), which is a basic physical quantity of the MgO tunnel barrier, has not been experimentally investigated due to the difficulty in measuring the ε r of small and thin MgO tunnel barriers. In this study, we measured ε r for a MgO tunnel barrier in epitaxial stacks and systematically investigated the relationships among ε r , epitaxial strain, and the VCMA effect. We showed that the MgO tunnel barrier in the epitaxial stacks was subjected to ~1.5% compressive strain and exhibited an enhanced ε r (>15), which is more than 50% greater than that of bulk MgO. Our results indicate that the compressive strain in the thin MgO tunnel barrier enhances ε r and thus the VCMA coefficient. Such strain engineering makes simple rocksalt tunnel barriers more attractive for spintronics applications.